how to handle java.lang.nullpointerexception

Question

Hi everyone I am really stuck, I keep getting the java.lang.NullPointerException. I tried to handle it in every possible place but I am not doing it successfully. It is homework.If you can look and give some feedback about the java.lang.NullPointerException it would be great. The exception occurs in Captain.handleProblem() and MalfucntionHandler.proccessMalfunction()

    public abstract class MalfunctionHandler 
    {

        MalfunctionHandler next;
        /**
         * severity is a type of Severity 
         */
        Severity severity;

        /**
         * @param description describes the severity of the problem
         */
        String description;


        /**
         * @param f file object  that refers to the log-silver.txt
         */
        File f = new File("log-silver.txt");

        MalfunctionHandler(Severity severity)
        {
                this.severity = severity;
        }
         public String getDescription()
        {
            if(description == null)
            {
                description = "No description available. Probably serious.";
            }
            return description;
        }

        final protected void processMalfunction(Malfunction malfunction)
        {
            if (this.severity == malfunction.getSeverity())
            {
               handleProblem();
            }
            else
            {
    //            if(malfunction == null)
                next.processMalfunction(malfunction);
            }
        }
        final protected void addHandler(MalfunctionHandler next)
        {
            this.next = next;
        }
        abstract void handleProblem();

        public Severity getSeverity() 
        {
            return severity;
        }
    }


public class Malfunction 
{
    /**
     * severity is a type of Severity 
     */
    Severity severity;

    /**
     * @param description describes the severity of the problem
     */
    String description;

    Malfunction(Severity severity, String description)
    {
        this.description = description;
        this.severity = severity;
    }

    public Severity getSeverity() 
    {
        return severity;
    }

     public String getDescription()
    {
        if(description == null)
        {
            description = "No description available. Probably serious.";
        }

        return description;
    }
}

public enum Severity 
{
     TRIVIAL, LOW, MEDIUM, HIGH
}

public class SpaceMonkey extends MalfunctionHandler {

    MalfunctionHandler malfunction;

    SpaceMonkey(Severity severity)
    {
       super(severity);
    }
    @Override
    void handleProblem() 
    {
        if(malfunction.getDescription() == null)
        {
            description = "No description available. Probably serious.";
        }
           FileUtility.writeFile(f, malfunction.getDescription()); 
           FileUtility.writeFile(f, "---> Space monkey assigned to problem.");
    }
}

public class ServiceRobot extends MalfunctionHandler {


     MalfunctionHandler malfunction;

    ServiceRobot(Severity severity)
    {
        super(severity);
    }
    void handleProblem() 

    {
       if(malfunction.getDescription() == null)
        {
            description = "No description available. Probably serious.";
        }
           FileUtility.writeFile(f, malfunction.getDescription());
           FileUtility.writeFile(f, "---> Service Robot assigned to problem.");
    }

}

public class Engineer extends MalfunctionHandler
{

     MalfunctionHandler malfunction;

    Engineer(Severity severity)
    {
        super(severity);

    }

    void handleProblem() 
    {
         if(malfunction.getDescription() == null)
        {
            description = "No description available. Probably serious.";
        }
          FileUtility.writeFile(f, malfunction.getDescription());
           FileUtility.writeFile(f, "---> Engineer assigned to problem.");
    }

}

public class Captain extends MalfunctionHandler
{
     MalfunctionHandler malfunction ;

    Captain(Severity severity)
    {
        super(severity);
    }

    @Override
   void handleProblem( ) 
    {
        if(malfunction.getDescription() == null)
        {
            description = "No description available. Probably serious.";
        }
           FileUtility.writeFile(f, malfunction.getDescription());
           FileUtility.writeFile(f, "---> Captain assigned to problem.");
    }
}

You never initialise MalfunctionHandler malfunction in the Captain class, so it is null and malfunction.getDescription() will throw a NPE - there might be other NPE in your code. — assylias
– assylias, Commented Nov 9, 2012 at 9:50

PermGenError · Accepted Answer · 2012-11-09 10:06:04Z

5

 if(malfunction.getDescription() == null)

you never initialized your MalfunctionHandler object in class SpaceMonkey and trying to call its getDescription() method in handleProblem method. In java Objects get default value as null your MalfunctionHandler malfunction; is null here and you are trying to access its method on null.

as your MalfunctionHandler is an abstract class, initialize it with its sub class (SpaceMonkey)

 MalfunctionHandler malfunction; = new SpaceMonkey(Severity);

edited Nov 9, 2012 at 10:06

answered Nov 9, 2012 at 9:55

PermGenError

46.5k8 gold badges90 silver badges107 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

PermGenError Over a year ago

@RohitJain MalfunctionHandler is an abstract class not Malfunction :)

Rohit Jain Over a year ago

Sorry I mean MalfunctionHandler. malfunction is a reference of that class.

PermGenError Over a year ago

@RohitJain no worries, BTW you rock mahn .. your answers are pretty informative .. :)

Doesn't Matter Over a year ago

But in my code it is not Malfunction malfunction and it is MalfunctionHandler malfunction

Patruni Srikanth · Accepted Answer · 2012-11-09 09:56:27Z

2

Malfunction object is not initialized, its just declared in the Captain class.

Also its not advisable to catch NullPointerExceptions. Instead you should validate and give checks in your code so that such exceptions won't be produced.

answered Nov 9, 2012 at 9:56

Patruni Srikanth

7691 gold badge7 silver badges15 bronze badges

Comments

someone · Accepted Answer · 2012-11-09 09:56:42Z

2

By default object type of instance variable are null

 MalfunctionHandler malfunction;

and

 MalfunctionHandler malfunction = null;

are same. Your classes has this problem

here if(malfunction.getDescription() == null) and malfunction is null so you are getting NPE here.

answered Nov 9, 2012 at 9:56

someone

6,5728 gold badges43 silver badges63 bronze badges

Collectives™ on Stack Overflow

how to handle java.lang.nullpointerexception

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related